The invention relates to static AC-to-DC power converters, such as can be used for AC or DC motor drive systems.
References Cited
U.S. Patent Documents
U.S. Pat. No. 5,455,759 October 1995 Paice
U.S. Pat. No. 5,619,407 April 1997 Hammond
U.S. Pat. No. 6,191,968 February 2001 Paice
Other Publications
xe2x80x9cPower Electronic Converter Harmonicsxe2x80x9d by Derek A. Paice, published 1995 by the IEEE Press, ISBN 0-7803-1137-X
To meet industry needs for electrical power converters which convert AC to DC without injecting large amounts of harmonic currents into the power system, several transformer and converter topologies are available. All require installation of extra equipment and add to the total cost. Preferred methods are those which perform well in practical power systems which incorporate voltage unbalance and preexisting harmonic voltages. Powerful techniques, in which three-phase power is converted to nine-phase power before processing, have been described in U.S. Pat. Nos.5,455,7595, 5,619,407 and 6,191,968 B1. Continual improvements are sought to reduce cost, particularly in systems with lower output current.
This invention describes a three-phase to nine-phase transformer with seven fewer windings than in U.S. Pat. No. 6,191,968 B1. A wider range of wire sizes is used in manufacture, but a savings in cost and weight is obtained, especially in lower HP ratings.
A three-phase to nine-phase auto-transformer with the main windings configured in a wye manner is arranged to provide nine output voltages which are of generally equal amplitude with a phase displacement of generally 40xc2x0. Such voltages applied to a nine-phase bridge rectifier arrangement result in low harmonic currents being drawn from the three-phase AC power source. Thus the rectifier equipment is much better able to conform to specifications such as IEEE 519 which restrict the amount of harmonic current which may be injected into an electrical power system. In its original form for 18-pulse operation as described in U.S. Pat. No. 6,191,968 B1 a single transformer with seven coils per phase provides nine output voltages of an amplitude such that the converter DC output voltage is generally the same as would be obtained with a three-phase rectifier bridge connected directly to the three-phase power source. By this means lower harmonic currents are feasible without affecting the basic design of existing conversion equipments. However, if required, higher or lower voltages can be achieved with suitable tappings on the transformer windings in a manner well known to those familiar with transformer design. A simplification to U.S. Pat. No. 6,191,968 B1 enables similar low harmonic currents to be obtained, but with a total of only 14 coils.
The preferred embodiment for 18-pulse operation with a total of 14 coils is shown in FIG. 3. It includes a basic wye structure auto-transformer, such as that taught in U.S. Pat. No. 6,191,968 B1 but includes only one delta path instead of six. For example, in FIG. 3, one coil labeled N7C is connected between terminals 2 and P1 to provide a path for 3rd harmonics of current.